Preparation of oils having improved oxidation stability



Feb. 21, 1967 l. W. MILLS PREPARATION OF OILS HAVING IMPROVED OXIDATIONSTABILITY Filed Nov. 20, 1964 300 40o OXIDATION TIME, HRS.

9 co Q) INVENTORS |VO R w. MILLS BY JOHN J. MELCHIORE cum 8.

ATTORNEY United States Patent Office 3,305,480 Patented Feb. 21, 19573,305,480 PREPARATION F ()lLS HAVING IMPROVED UXHDATION STABILITY IvorW. Miils, Glenolden, and John J. Melchiore, Wallingford, lla.,assignors' to Sun Oil Company, Philadelphia, Pa., a corporation of NewJersey Filed Nov. 20,1964, Ser. No. 412,678

4 Claims. (Cl. 208-273) 1 This invention relates to the preparation ofhydrocarbon oils having improved oxidation stability and particularlyimproved resistance to oxidation in the presence of copper and iron. 7The invention is applicable generally to the preparation of improvedhydrocarbon distillate oils boiling above 500 F. but is especiallyuseful for improving oils intended for use in electrical systems, suchas transformers and capacitors, and also refrigerator oils.

It is well known that the presence of copper in petroleum distillatescan have an adverse effect on oxidation stability of the hydrocarbonsduring use. Certain oils, such as transformer oils, cable oils,refrigerator oils and the like, are normally used in systems in whichcopper is present, and the copper can have a pronounced effect inaccelerating deterioration of the oil through oxidation. Other petroleumdistillate oils, while not necessarily being intended for use incopper-containing systems, often contain deleterious amounts of copperdue to contact with copper-containing equipment during processing andsubsequent handling. Thus copper can be present in suflicient amount topromote oxidation and result in gum or sludge formation in suchdistillates as furnace oils, diesel oils and lubricating oils. Theaccelerated oxidation of these products may give rise to sludging instorage tanks, clogging of filters or nozzles, corrosion of equipment,loss of desirable product qualities such as dielectric strength, and

other undesirable effects.

The present invention provides a novel method of producing frompetroleum distillate oils boiling above 500 F. refined oil productswhich have exceptionally good oxidation stability in the presence ofcopper and also iron. While the method has general applicability to thetreatment of distillate oils boiling in the gas oil and lubricatingranges, it is especially useful for refining oils which have viscositiesin the range of 50-250' SUS at 100 F. These include transformer oilswhich generally have viscosities in the range of 50-65 SUS at 100 F.,capacitor oils of 90-110 SUS at 100 F. and refrigerator oils whoseviscosities usually fall in the range of 140-220 SUS at'100 F.

Charge oils used in practicing the present invention have aromaticcontents of at least 10% by weight and usually have a considerablyhigher content of aromatic hydrocarbons. Untreated distillate oils fromnaphthenic crudes generally have aromatic contents in the range of30-50% by weight, while those from paraffinic crudes after being dewaxedusually contain 20-30% aromatics. These distillate oils can the solventextracted prior to treatment according to the present invention and thisis often preferable. Extraction with a solvent such as furfural usingconventional extraction conditions usually will reduce the aromaticcontents of the napht-henic distillates to 20-35% and of the dewaxedparafiinic distillates to 10-20%. These oils, whether solvent extractedor not, invariably contain an appreciable amount of sulfur'and nitrogencompounds.

The method according to the present invention involves first treating acharge oil as described above with a sulfonating agent and utilizingsevere treating conditions such that the aromatic content of the treatedoil is less than by weight, more preferably less than 2% by weight. Inother words this severe sulfonation treatment produces a refined oil ofthe type generally referred to as a technical grade white oil, which notonly has a low aromatic content but also only trace or negligibleamounts of constituents containing sulfur and nitrogen. After separatingsulfonated material from the treated oil, the latter is thencatalytically dehydrogenated under conditions such that thedehydrogenated product boiling above 500 F. will have an aromaticcontent in the range of 15-40% by weight. This product will haveoutstanding stability characteristics and resistance to oxidation in thepresence of metals such as copper and iron.

in the sulfonation step of the present process the charge oil containingin excess of 10% aromatics is treated with either fuming sulfuric acid(oleum) or sulfur trioxide employing treating conditions which are wellknown in the art for producing white oils. With either oleum or sulfurtrioxide as the sulfonating agent, the treatment conventionally iscarried out in a series of steps in each of which a portion of the totalsulfonating agent is contacted with the oil. After each contacttreatment the reaction mixture is settled. The resulting upper oil layerand lower acid sludge layer are separated from each other, and the oillayer is then treated with another portion of the sulfonating agent.After sufficient'successive treats have been carried out to reach thedesired white oil quality, the acidic oil is extracted with aqueousalcohol and then neutralized with caustic soda to remove all remainingsulfonated material from the oil.

Sulfonating procedures for producing white oils are described in Ind.and Eng. Chem, vol. 49, pages 31-38, January 1957, and in Lipkin et al.US. Patent No. 2,680,- 716 and need not be described in further detailherein.

The treated oil free of sulfonatedmaterial next is catalyticallydehydrogenated under conditions to effect substantial conversion ofnaphthcnic components to aromatics. Conditions of conversions are suchas to avoid hydrocracking while effecting dehydrogenation of naphthenesto an extent such that the dehydrogenated product boiling above 500 F.will have an aromatic content of at least 15% by weight. The aromaticcontent of the dehydrogenated product may range up to 40% depending uponthe naphthenic character of the original charge stock and the degree ofdehydrogenation effected. Generally a small amount of product boilingbelow 500 R, will be produced via hydrocracking reactions but the degreeof hydrocracking can be minimized by appropriate selection of reactionconditions.

The dehydrogenation reaction is carried out by contacting the treatedoil in a hydrogen atmosphere with a dehydrogenation catalyst.Temperature and pressure conditions that should be used will depend uponthe particular catalyst selected but in general will be in the ranges,re-

spectively, of 650-825" F. and -750 p.s.i.g. The specific temperatureand pressure conditions should be correlated with regard to the activityof the particular dehydrogenation catalyst selected so as to minimizecracking while effecting dehydrogenation to a degree whereby the productwhich boils above 500 F. will have between 15% and 40% aromaticconstituents. As a general rule the specific temperature selected withinthe above-specified range of 650-825 F. should vary inversely with theactivity of the catalyst employed.

A preferred catalyst for use in the dehydrogenation step isplatinum-on-alumina in which no halogen component has been incorporated.This catalyst is highly active for dehydrogenation of naphthenes andgenerally can be used at lower temperature than can other conventionaldehydrogenation catalysts. While platinum catalysts normally aresensitive to sulfur, this presents no problem in the present processsince the treated oil from the sulfonation step will have a negligiblesulfur content.

Typical conditions for using a platinum-on-alumina catalyst include atemperature of 650-775 F., a pressure of 100-400 p.s.i.g., a hydrogenrecycle ratio of 2000-10,000 s.c.f./bbl. of charge oil and an LHSV of0.5-2.

Other dehydrogenation catalysts that can be used include nickelsulfide-molybdenum sulfide-on-alumina and nickel sulfide-tungstensulfide-on-alumina. Typical conditions for using either of thesecatalysts include a temperature of 750-800 F., a pressure of 400-500p.s.i.g., a hydrogen recycle ratio of 4000-10,000 s.c.f./bbl. of oil andan LHSV of 0.5-2.0.

Another commercially available catalyst that can be used for the presentpurpose is cobalt oxide-molybdenum oxide-on-alumina, often referred toas cobalt molybdate catalyst. This catalyst is somewhat less active thanthose mentioned above. Typical conditions for its use include atemperature of 775-810 F., a pressure of 350-450 p.s.i.g., a hydrogenrecycle ratio of 5000- .000 s.c.f./bbl. of oil and an LHSV of 0.25-1.25.

The dehydrogenated product formed under conditions as described abovegenerally will contain a minor amount of material boiling below 500 F.as a result of some amount of hydrocracking occurring during thereaction. Usually this lower boiling material will amount to between 1%and 10% by volume of the total product, although under the more severereaction conditions the amount may range up to 20%. The product obtainedupon removal of this lower boiling material will contain between and 40%by weight of aromatic hydrocarbons. It is usually preferable tocorrelate and adjust the reaction conditions in the dehydrogenation stepso that the product boiling above 500 P. will have an aromatic contentin the range of -35%. This product will have exceptionally goodoxidation stability in the presence of copper and/or iron. If desired,the product can be distilled into fractions of any selected boilingranges each of which fractions likewise will have outstandingly goodoxidation stability in the presence of such metals.

For the purpose of illustrating the invention more specificaly, theinvention will be described with reference to the prepartion oftransformer oils. Such oils generally boil in the range of 500-775 F.and have viscosities in the range of 50-65, more preferably 55-60, SUSat 100 F. A list of typical characteristics of commercial transformeroils is given in the text by F. M. Clark entitled Insulating Materialsfor Design and Engineering Practice (1962), page 135. The presentdescription is presented in conjunction with the accompanying sheet ofdrawings in which curves are shown which represent, respectively, aconventional commercial transformer oil, a white oil made from atransformer stock and a dehydrogenated oil prepared from such white oilin accordance with the invention, all as more fully describedhereinafter.

Commercial transformer oils customarily are tested by the DobleOxidation Test for electrical insulating oils developed by the DobleEngineering Company of Belmont, Massachusetts. This procedure has beendescribed in ASTM Standards on Electrical Insulating Liquids and Gases,pages 307-313, December 1959, under the title Suggested Method of Testfor Oxidation Characteristics of Mineral Transformer Oil. It involvesbubbling air through a known amount of the oil held at a temperature of95 C. in the presence of copper and iron and making two types of testsdaily on small samples of the oil. One type of test is an aciditymeasurement. The other is a precipitation test in which one volume ofthe oil is diluted with five volumes of pentane, the mixture is allowedto stand at least eight hours and the presence or absence of a sludgeprecipitate is noted. The endpoint of the Doble test is taken as thenumber of days of oxidation either before the acidity of the oil reaches0.25 mg. of KOH per gram or before a positive precipitation test forsludge is obtained. Commercial transformer oils heretofore availableusually have a life of only about threedays under Doble test conditions.Longer life values could be obtained by adding an oxidation inhibitor tothe oil but the use of such additive traditionally has been consideredunacceptable by transformer manufacturers and users.

An amplification of the Doble test recommended by the Doble EngineeringCompany is the so-called Power Factor Valued Oxidation (PFVO). Thisinvolves operating in the manner described above but also determiningthe power factor of the oil at two hour intervals throughout theoxidation period. A curve is obtained by plotting the power factoragainst the oxidation time. The three curves shown in the accompanyingdrawing are curves obtained in this manner.

Curve A is a typical PFVO curve representing commercial transformer oilprepared by conventional treatment of a transformer oil stock withsulfuric acid and clay. It can be seen that a sharp hump in the PFVOcurve occurs in the earlier stages of oxidation. Thereafter the powerfactor begins to rise, and after an oxidation period of hours a rapidincrease in power factor is exhibited. However, usually before 100 hourshas been reached, the oil has failed in the Doble test due to sludging.Failure generally occurs at about three days (72 hours).

The preparation of the oils corresponding to curves B and C of thedrawings is described in the following example which constitutes aspecific embodiment of the invention.

EXAMPLE A paraffin distillate stoc-k boiling in the approximate range of570-740 F. and having a viscosity of 57 SUS at 100 F. was dewaxed toform a charge oil having a pour point of about 15 R, an aromatic contentof about 25% by weight, a nitrogen content of 25 p.p.m. and a sulfurcontent of 0.2%. This charge oil was treated at about 80 F. for tensuccessive times using 20% oleum each time in amount of 20 lbs./bbl. ofcharge oil. After each treat the mixture was settled and the acid sludgelayer was separated from the oil. After the tenth treat with oleum theacidic oil was extracted with an equal volume of 30% aqueousisopropanol, and then was Water washed followed by a caustic soda wash.Finally the treated oil was dried by contacting it mm 2 lbs./bbl. ofadsorptive clay.

The foregoing sultfonation treatment produced a technical grade whiteoil having an aromatic content of 0.3%, a sulfur content of 25 ppm. anda negligible 1 p.p.m.) nitrogen content. In the Doble Oxidation Testthis oil had a life of 5 days, with acidity build-up being the limitingcriterion. The PFVO curve for this oil is curve B in the drawing. Fromcurve B it can be seen that this oil was somewhat better than thetypical commercial oil illustrated by curwe A. The white oil had goodinsulating characteristics as shown by low power factor but only forabout hours. Thereafter a sharp rise in the power factor occurred andthe oil was no longer suitable as a transformer oil.

The technical grade white oil was then dehydrogenated in a flow reactorin a hydrogen atmosphere maintained in the usual manner by recyclingefiluent hydrogen while adding fresh make-up hydrogen. Aplatinum-on-alumina catalyst substantially free of halogen was employed.Reaction conditions used comprised a temperature of about 775 F.,pressure of about 150 p.s.i.g., LHSV of 1.0 and a hydrogen recycle ratioof 10 moles per mole of white oil charged. The dehydrogenation productcontained about 5% hydrocarbons boiling below 500 F. which wereremoved'by distillation.

The dehydrogenated oil thus obtained had an aromatic content of 32.5% byweight, which was composed of 23.8% mononuclear, 6.0% dinuclear and 2.7%trinuclear aromatics. Carbon type analysis of the oil showed 11%aromatic carbons, 20% naphthenic carbons and 69% parafiinic carbons. Theoil had 16 ppm. of sulfur. and essentially no nitrogen constituents.

In the Doble Oxidation Test the dehydrogenated oil showed aninordinately long life, namely, 25 days. The PFVO curve is curve C whichshows that that the dehydrogenated oil had outstanding stability asmeasuered by power factor. From curve C it can be seen that an upwardbreak in the PFVO curve did not ocur until the oxidation time hadexceeded 950 hours.

The foregoing example shows that the present invention can producerefined transformer oil which has outstanding oxidation stability in thepresence of copper and iron. When the invention is used to refine otherstocks such as capacitor oils, cable oils, refrigerator oils and thelike, analogous improvements in the oil stability are effected.

We claim:

1. Method of preparing an oil having improved oxidation stability in thepresence of copper which consists of:

(a) treating a petroleum distillate oil boiling above 500 F, having aviscosity in the range of 5065 SUS at 100 F. and having an aromaticcontent of at least by weight with a sulfonating agent selected from thegroup consisting of fuming sulfuric acid and sulfur trioxide in amountto reduce the aromatic content of the oil to less than 5% by weight,

(b) separating sul-fonated material from the treated oil,

(0) catalytically dehydrogenating the treated oil to an extent such thatthe dehydrogenated product boiling above 500 F. has an aromatic contentin the range of -40% by weight,

(d) and recovering the dehydrogenated product boiling above 500 F. assaid oil having improved oxidation stability.

t5 2. Method according to claim 1 wherein the amount of sulfonatin gagent used is sufficient to reduce the aromatic content to less than 2%by Weight.

3. Method of preparing an oil having improved oxidation stability in thepresence of copper which consists of: (a) treating a petroleumdistillate oil boiling above 500 F., having a viscosity in the range of65 SUS at 100 F. and having an aromatic content of at least 10% byweight with a sulifonating agent selected from the roup consisting offuming sulfuric acid and sulfur trioxide in amount to reduce thearomatic content of the oil to less than 5% by weight, (b) separatingsulfonated material from the treated oil, (c) dehydrogenating thetreated oil by contacting the same in a hydrogen atmosphere with adehydrogenation catalyst under dehydrogenating conditions including atemperature in the range of 650825 F. and a pressure in the range of100750 p.s.i.g., said conditions being correlated such that thedehydrogenated product boiling above 500 F. has an aromatic content inthe range of 15-40% by weight, (d) and recovering the dehydrogenatedproduct boiling above 500 F. as said oil having improved oxidationstability. 4. Method according to claim 3 wherein the amount ofsulfonating agent used is sufiicient to reduce the aromatic content toless than 2% by weight.

References Cited by the Examiner UNITED STATES PATENTS 5/1961 De Chellis et al 208- 9/1961 Murray et al 20 8-90

1. METHOD OF PREPARING AN OIL HAVING IMPROVED OXIDATION STABILITY IN THEPRESENCE OF COPPER WHICH CONSISTS OF: (A) TREATING A PETROLEUM DISTILATEOIL BOILING ABOVE 500*F., HAVE A VISCOSITY IN THE RANGE OF 50-65 SUS AT100*F. AND HAVING AN AROMATIC CONTENT OF AT LEAST 10% BY WEIGHT WITH ASULFONATING AGENT SELECTED FROM THE GROUP CONSISTING OF FUMING SULFURICACID AND SULFUR TRIOXIDE IN AMOUNT TO REDUCE THE AROMATIC CONTENT FO THEOIL TO LESS THAN 5% BY WEIGHT, (B) SEPARATING SULFONATED MATERIAL FROMTHE TREATED OIL, (C) CATALYTICALLY DEHYDROGENATING THE TREATED OIL TO ANEXTENT SUCH THAT THE DEHYDROGENATED PRODUCT BOILING ABOVE 500*F. HAS ANAROMATIC CONTENT IN THE RANGE OF 15-40% BY WEIGHT, (D) AND RECOVERINGTHE DEHYDROGENATED PRODUCT BOILING ABOVE 500*F. AS SAID OIL HAVINGIMPROVED OXIDATION STABILITY.